Hybrid structures made of a backbone member and a secondary member which are of different materials are used in the manufacture of high load-bearing applications, such as frames and chassis in the automotive, furniture, and computer industries, for example. A hybrid structures can be made of an open or a closed cross-section backbone member, and a secondary member. The backbone member is often made of metal which provides strength and rigidity, and the secondary member is often made of polymer which can provide strength and functionality. The use of a closed cross-section backbone provides a relatively strong component where the hybrid structure is made, for example, by joining or over-molding a polymeric secondary member to a metal tube that has been shaped by hydroforming. Hybrid structures can also be made using open cross-section backbone members and can allow for design flexibility, such as variation in the width of the backbone member along its length, however, these open structures will have limited torsional and load-carrying capability.
Although the closed cross-section hybrid structures provide for higher torsional rigidity and load-carrying capacity, such hybrid structures can have limitations in achieving cross-section re-shaping and routing of the backbone into complex shapes, for example, in applications which require high bend radii. Another disadvantage of the closed cross-section backbone structure is the limitation on wall thickness and depth to which the backbone can be shaped or drawn while maintaining stable strain distribution throughout the hybrid structure. Not only does this place a constraint in the design of the hybrid structure for an end-use application, but can also result in excess weight, for example, in a hybrid structure made from a tube having a greater wall thickness to prevent wall-thinning in portions of the backbone member during hydroforming.